The present invention relates to a system and a method for adaptive control of a lock-up clutch in a hydrokinetic torque transmitting unit.
Japanese patent application First Publication (unexamined) No. 63-172058 discloses a system for a control of a lock-up clutch in a hydrokinetic torque transmitting unit including a pump impeller, and a turbine runner. When it is engaged, the lock-up clutch connects the turbine runner to the pump impeller. The system comprises an electro-hydraulic circuit, including an electric actuator in the form of a lock-up solenoid, for hydraulically controlling engagement of the lock-up clutch, and a control unit for generating an operating signal supplied to the electric actuator. The duty of the lock-up solenoid is varied by the operating signal generated by the control unit. When duty of the lock-up solenoid is zero, the lock-up clutch is released. After the control unit starts generating the operating signal, the duty of the lock-up solenoid gradually increases since the operating signal is indicative of the duty determined in the control unit. After a time interval, engagement of the lock-up clutch is initiated, and when the duty attains a predetermined value, the lock-up clutch is fully engaged. Describing this process of engagement in other words, a slip in the hydrokinetic torque transmitting unit starts decreasing after the initial engagement of the lock-up clutch.
In the case where a fluid coupling with a lock-up clutch is used as a hydrokinetic torque transmitting unit, it is the common practice to engage the lock-up clutch at a very low vehicle speed immediately after the motor vehicle has started moving from a standstill. If this lock-up process is conducted by controlling the duty of a lock-up solenoid in such a manner as to decrease a difference between an actual slip in the fluid coupling and a target slip, there is the tendency that a rapid increase in engine speed occurs to bring an actual slip in the fluid coupling into agreement with a target slip since a revolution speed of a turbine runner stays very slow immediately after the vehicle has moved from a standstill. This increase in engine speed is felt by a driver of the vehicle as engine racing.
The present invention aims at improving a control of a lock-up clutch in a hydrokinetic torque transmitting unit such that, during engagement of the lock-up clutch immediately after the motor vehicle has moved from a standstill, variation of engine speed is sufficiently smooth.